Liquid Crystal

Liquid crystals viewed under polarized light.

Discovered more than 100 years ago, liquid crystals still remain an area of research today.

History

Friedrich Reinitzer

Many scientists had already observed this phenomenon of liquid crystals prior to its "discovery", however the Austrian botanist Friedrich Reinitzer is more often then not given credit for its discovery. While experimenting with cholesterol benzoate in 1888, Reinitzer noted that it the substance had two melting points. The first melting point occurred at 145.5°C, where it melted to a cloudy liquid phase, and 178.5°C, where it melted to a clear liquid phase. He also observed some strange color phenomena as the temperature changed.

Reinitzer then sent some of his samples to a German physicist, Otto Lehmann, who had a state of the art polarized microscopic, which he outfitted with heating stage so he could precisely control the temperature of his specimens. Lehmann first described Reinitzer's sample as floating crystals and crystalline fluids, but after more experimentation became convinced that the cloudy liquid was a uniform phase all its own. This led him to coin the term liquid crystal to describe this new phase of matter.

In the early 1900s, many advances were made in the understanding liquid crystals. In 1922, French physicist, Georges Freidel, gave a detailed description of different liquid crystal phases, coining many terms that are still used today. Freidel also explored the orienting effect of an electric field on liquid crystals.

After World War II interest in liquid crystals began to wane. Many scientists believed that all of the important problems regarding liquid crystals had been solved. However, in the 1960s scientists began to re-examine liquid crystals and continued the progress of the pre-war era. It was discovered that liquid crystalline substances were extremely sensitive to very small changes in temperature. Researchers at RCA found that with an electrical voltage applied, a thin layer of liquid crystal was capable of switching from cloudy to clear.

Overview

• Insert Chart Collings pg. 11

Solids possess both positional order, where molecules are constrained to occupy only certain positions, and orientaitional order, where each molecule is oriented with respect to one another. When a material transitions to a liquid both the positional and orientational order are lost, and the material becomes isotropic, or uniform in all directions. In the liquid crystal phase, molecules, or mesogen, display a wide range of positional and orientational orders, leading to several subphases. These subphases are defined by the amount of order present in the liquid crystal.

Liquid crystals are a phase of matter between the solid phase and the liquid phase. If you have a material that has a liquid crystal phase you would observe the two melting points that Reinitzer observed, one from the solid to liquid crystal transition and the second from the liquid crystal to isotropic liquid transition.

However, this is only the case in thermotropic liquid crystal, where phase transitions are dependant on the temperature. It is possible for liquid crystal phase to be dependant on the concentration of component in another, this is called lyotropic liquid crystal.

Some molecules are more likely to posses a liquid crystal phase than others. There are 3 properties that make a molecule a good candidate to become liquid crystal:

(1) Elongated in shape: elongated molecules have stronger attractive forces when they are aligned parallel to one another.

(2) Rigid in the center:

(3) Flexible on the ends: allows for easier positioning in between other molecules

The rod shaped liquid crystal is the most common type, however there are others. In 1977, disclike molecules were discovered that formed a liquid crystal phase. This type of liquid crystal is known as discotic.

Various subphases of discotic liquid crystal.

Since the liquid crystal phase is between that of solid and liquid, it displays characteristics common to both, as well as some unique characteristic all its own. Molecules are free to move about as in a liquid, however they on average spend more time pointing along the direction of orientation. Liquid crystal will also flow and take the shape of its container, however it also display a cloudiness which suggests that it is different from the liquid phase. Many unique characteristics of liquid crystals stem from the fact that it is an anisotropic phase. This means that the properties of the material are dependent on the direction that it is oriented in.

Methods to Measure Orientational Order

Distribution function of the order parameter, S. Zero represents complete disorder, while one represents complete order.

To quantitatively describe the amount of orientational order present in a subphase of liquid crystal the average direction of the mesogen, is measured with respect to a direction of orientation known as the director. Each molecule makes an angle, $\theta$ with respect to the director. This angle is then measured and averaged over all $n$ molecules.

${\bar {\theta }}=\sum _{i=0}^{n}{\frac {\theta _{i}}{n}}$

The more orientational order present the closer the average will be to zero. An alternative way to do this is to take the average angle of one molecule over a certain time, instead of looking at different molecule. However, this only works under the assumption that all molecules undergo random motion. Thankfully, this assumption has never been contradicted experimentally.

To create a more standard method of measuring orientational order we introduce the order parameter $S$. Where once again we take the angle $\theta$ with respect to the director, however this time we use the function,

$S=<{\frac {3\cos ^{2}{(\theta )}-1}{2}}>$

Now integrating over the solid angle,

$S=\int {\frac {3\cos ^{2}{(\theta )}-1}{2}}f(\theta )d\Omega$

where $f(\theta )d\Omega$ is the fraction of molecules in a solid angle $d\Omega$ which are oriented at an angle of $\theta$ to the director.

Now perfect oriental orientation will have an average of 1 and no orientational order will have an average of 0.

Subphases

Although many compounds exhibit only one liquid crystal phase, it is not unusual for a single substance to posses more than one. To the right are phase diagrams of three different molecules that form into different subphases at different temperatures.

Nematic

Nematic Phse can be thought of as a liquid that has long range orientational order, but lacks postional order.

Smectic

Positionally ordered in 1D and orientational order

Smectic A

Smectic C

Chirality

• Latent Heat shows that liquid crystal is closer to a liquid than it is to a solid.

Influences of an External Field

One of the reasons liquid crystal has so many applications is because of its interactions with an external field, be it electrical or magnetic. Solids, liquids, and gases respond to electric and magnetic fields, but the response is minimal even when strong fields are applied. Liquid Crystal, on the contrary, respond to even weak electric and magnetic fields with significant structural change.

Applications

• Many common liquids are liquid crystals (i.e. soap)

Displays

In a simple Liquid Crystal Display (i.e. no color), there are two polarizers placed a small distance apart from each other. The polarizers are set up, so that their angles of polarization are perpendicular to each other. This means if light were to pass from the first straight through to the second then an observer would not see any light through the polarizer.

Now a thin layer of liquid crystal is placed in between the polarizers. With the liquid crystal in between the polarizers an observer would notice that light is now showing through the polarizers. This is because the liquid crystal is anisotropic and essentially acts as a polarizer itself and polarizes the light at some intermediate angle between that of the two surrounding polarizers. Now we can apply a force field (electric, magnetic, etc...) to the liquid crystal to align the particles in a direction that lets the light pass through unaffected. This means that again the observer will see no light.

So using this method we are able to insert electrodes into the area of the liquid crystals to create a localized electric field that will block out any light in that area, creating a black image on the observer's screen.

When the switch is closed an electrical field is created which adjusts the direction of the director to be orthogonal to the polarizers. The light is then able to pass unaffected through the liquid crystal and is absorbed completely by the second polarizer.

Liquid crystal displays, such as televisions and computer monitors, use the same process to create the images, however color filters are also used. Here each picture element, or pixel, consists of a red, green, and blue color filter. Different colors are then synthesized by varying the brightness of light through each one of these filters.

Takaki: 3D Display

A Novel 3D Display Using an Array of LCD Panels (2003), Yasuhiro Takaki, Tokyo University

Insert Figure 6

• No need for 3D Glasses

• No restriction on observing position

• No fatigue

Thermometer

• NASA Applications

• Mood Ring

References

Chandrasekhar, Liquid Crystals, 1977

Chigrinov, Liquid Crystal Devices: Physics and Applications, 1999

[1]Collings, Liquid Crystals, 1990

Jones, Soft Condensed Matter, 2002

[2]Kumar, Liquid Crystals, 2001

Saeva, Liquid Crystals: The Fourth State of Matter, 1979

Takaki, A Novel 3D Display Using an Array of LCD Panels, 2003

External Links

• Liquid Crystal Institute at Kent State University